Cryogenic cooling apparatus

ABSTRACT

A cryogenic cooler includes a heat exchanger having a helically wound finned tube of which the interior forms one path in which refrigerant gas from a supply under pressure is supplied to a Joule Thomson expansion nozzle, whence the low pressure gas returns through the other path outside the helical tube, and inner container of metal in which the heat exchanger and nozzle are permanently secured, a Dewar flask in which the inner container is removably fitted, and, between the outside surface of the container and an opposed inside surface of the flask, a fluid or paste heat transfer material of high heat conductivity.

Unite 11 tates Patent 11 1 1111 3,747,365

Nicholds July 24, 1973 [54] CRYOGENIC COOLING APPARATUS 3,424,230 1/1969 Wright 62/514 [75] In Kenneth mu Nicholds, 3,517,525 6/1970 Campbell 62/514 Redditch, Worcestershire, England i Primary Examiner-Meyer Perlm 1 1 Asslgneei The Hymatic Engineering Company Attorneye-watson, Cole, Grindle & Watson Limited, Redditch, England [22] Filed: Feb. 17, 1971 57 ABSTRACT Appl. No.: 1 16,150

A cryogenic cooler includes a heat exchanger having a helically wound finned tube of which the interior forms one path in which refrigerant gas from a supply under pressure is supplied to a Joule Thomson expansion nozzle, whence the low pressure gas returns through the other path outside the helical tube, and inner container of metal in which the heat exchanger and nozzle are permanently secured, a Dewar flask in which the inner container is removably fitted, and, between the outside surface of the container and an opposed inside surface of the flask, a fluid or paste heat transfer material of high heat conductivity.

1 Claim, 1 Drawing Figure CRYOGENIC COOLING APPARATUS This invention relates to cryogenic cooling apparatus including a generally tubular heat exchanger affording two paths in one of which refrigerant gas from a supply under pressure is supplied to a Joule Thompson expansion nozzle, whence the low pressure gas returns through the other path. Preferably a valve member cooperates with the nozzle to vary its effective area automatically for controlling the flow of refrigerant.

Various forms of such apparatus are known to those skilled in the art to which the present invention pertains.

According to the present invention the cooling apparatus includes an inner container of metal in which the heat exchanger and nozzle are permanently secured, a Dewar flask in which the inner container is removably fitted, and, between an outside surface of the container and an opposed inside surface of the flask, a fluid or paste heat transfer material of high heat conductivity.

In apparatus of the type referred to as hitherto constructed it has been customary to fit the heat exchanger direct into the Dewar flask, so that a close fluid-tight fit is required between the heat exchanger and associated parts, and the inner wall of the Dewar flask. The close tolerances required add to the difficulty of manufacture. In addition since the heat exchanger cannot in practice be permanently secured in the Dewar flask the various parts of the Joule Thompson cooler are liable to damage in handling, particularly bearing in their very small size. For example the internal diameter of the Dewar flask may be no more than some 7 millimeters or even less, and within this is contained a heat exchanger including a helical coil of finned tubing and a minute orifice forming the seating for a regulating valve.

The invention may be put into practice in various ways, but one specific embodiment will be described by way of example with reference to the accompanying drawing in which the single fiqure is a sectional elevation of a cooling apparatus working on the Joule Thompson principle.

in the embodiment shown the cooling apparatus, like most of those of the specifications referred to above, is of elongated form, and will be described in the position in which it would normally be used with its axis vertical and its cold end at the bottom. The heat exchanger, orifice valve and sensor are not inserted directly into a Dewar flask, rather, they are encapsulated in a metal can or inner container which is permanently secured to them, as by welding. Thus the apparatus includes a tubular heat exchanger comprising an inner tubular body around which is helically wound a finned inlet tube 11 forming the inlet path of the heat exchanger.

In accordance with the present invention a metal can 9 preferably of stainless steel, is located round the finned coil 11 and the space between the inner body 10 and the can 9 provides the second or exhaust path of the heat exchanger for exhaust gas flowing past the fins to cool the incoming high pressure refrigerant within the helical coiled tube 11 forming the inlet path. The upper end of the can 9 is welded or brazed to the upper end or head 15 of the tubular body 10 while its lower end is closed to provide a reservoir in which liquid working fluid can accumulate. The upper end of the he lical finned tube 11 communicates with a central bore in the head 15 of the body to which working fluid under pressure is supplied at a temperature below its inversion temperature.

At its lower end the inner tubular body 10 has welded to it a reinforcing ring 16 having a sensor 17 projecting parallel to the axis from one point of it as described below, and having, projecting parallel to the axis from a diametrically opposite point, a threaded stud 20 for mounting a seating member 24.

The seating member 24 comprises a stout disc 25 one face of which has projecting eccentrically from it a part-circular boss 26 from which in turn a smaller circular boss 27 projects still further. The disc has in it a hole which receives the threaded stud 20, and is held in place by a nut 21. The small circular boss 27 projects coaxially up into the cold end of the heat exchanger, whilst the part-circular boss 26, which may comprise approximately a semi-circle, is also coaxial with the heat exchanger and fits snugly into the reinforcing ring 16. The small circular boss 27 of the seating member 24 has a coaxial bore 28 extending through it from its upper end to a valve orifice 29 opening through its lower end, and a transverse bore 30 which opens into the axial bore 28 and contains a filter 31, and of which the outer end is closed by a screw plug (not shown). A

further transverse bore (not shown) opens into this transverse bore 30 and the lower end of the helical heat exchanger tube is sealed into this last transverse bore. The upper end of the axial bore 28 is closed.

The effective area of the expansion nozzle 29 is arranged to be controlled by means of a needle valve 34 which is itself controlled by a bellows 35.

The bellows 35 has its lower open end secured to the reinforcing ring 16 whilst its movable closed upper end is secured to the upper end of a depending tube 36, which will be referred to herein as a hollow piston rod. This extends down beyond the seating member 24 and half the circumference of its lower portion is cut away whilst the remaining half receives and is secured to and reinforced by a tubular valve carrier 37. The valve carrier is also cut away for half its circumference except at its end portion. Thus the seating member 24 projects into the open half of the hollow piston rod 36 and the valve carrier 37 from the side, and the small cylindrical boss 27 of the seating member 24 projects up into and fits in the upper end of the valve carrier to guide it. The lower end of the valve carrier carries the needle valve 34 which has a lower cylindrical portion and an upper tapered portion projecting into the expansion orifice 29 of the seating member 24.

As referred to above the reinforcing ring 16 in the lower end of the tubular body 10 of the heat exchanger carries a sensor 17. This is in the form of a metal tube 18 sealed in a hole extending through the reinforcing ring 16 parallel to the axis and having its lower end portion squashed flat to form an extended heat conducting tail 19. The sensor tube 17, and the space outside the bellows 35 inside the tubular body 9 of the heat exchanger, are filled with liquid and vapor in equilibrium of a suitable material, which may or may not be the same as the refrigerant.

Thus in operation, as described above, as the liquid refrigerant collects in the vessel 9 and the level of the pool of liquid gradually rises, progressively immersing the extended tail 19 of the sensor 17, the temperature of the sensor tube progressively falls, the pressure applied to the outside of the bellows 35 falls correspondingly, and the bellows expands, raising the hollow piston rod 36 and causing the needle valve 34 to progressively close the expansion orifice 24 so as to reduce the flow of refrigerant.

These minute, and hence rather delicate, parts of the cooling apparatus itself are permanently encapsulated in and protected by the metal can or inner container 9.

The bottom end of the can 9 is filled with absorbent material 40 such as felt or cotton-wall to absorb liquid refrigerant, which is caused to impinge on it. The absorbent material is covered by a flat disc 50 of wire gauze, which is secured to the can and has in it a hole 51 through which the sensor projects into contact with the absorbent material.

Accordingly in operation whatever the attitude and effective gravity of the cooler the mist of refrigerant liquid and gas projected from the nozzle, by a pressure of perhaps 6,000 pounds per square inch, is directed into the absorbent material.

As the absorbent material becomes saturated with liquid refrigerant, being in contact with the inner wall of the Dewar vessel, it provides effective cooling for the load on the other side of that wall. The disc ensures that droplets of refrigerant cannot by-pass or bounce back from the absorbent but are absorbed into it by capillary action.

The effect of the absorbent material in cooling the sensor is very much less when the absorbent material is dry than when it is saturated with liquid refrigerant and accordingly the operation of the valve depends sensitively on the extent to which the absorbent is saturated with liquid refrigerant and is to a large extent independent of the attitude of the cooler.

The metal can 9 is inserted into the interior of a Dewar flask 13. This carries the load to be cooled, for example a radiation detector 14 may be formed or applied to the outer surface of the inner wall of the Dewar flask, within the vacuum space. Accordingly there is no necessity for a fluid-tight fit between the metal can and the Dewar flask but on the other hand there must be a path for transfer of heat across the space between the outer surface of the bottom end of the metal can and the inner surface of the end of the inner wall of the Dewar flask. To provide this the space in question is occupied by a fluid or paste heat transfer material such as high heat conductivity grease 38. Various materials may be employed for this purpose but a particularly suitable one had been found to be a silicone-based grease such as that sold as Heat sink compound No. D.P.2623 by Midland Silicones Limited.

Thus the invention provides a form of cooling appa ratus in which manufacturing tolerances are eased and the working parts are protected from damage in handling by being encapsulated so as to form what may be termed an encapsulated cold finger.

What we claim as our invention and desire to secure by Letters Patent is:

1. Cryogenic cooling apparatus comprising:

a generally tubular heat exchanger providing two paths, in one of which refrigerant gas from a supply under pressure is supplied to a Joule Thomson expansion nozzle and in the other of which the low pressure gas is returned;

a valve member cooperating with said nozzle to vary its effective area automatically for controlling the flow or refrigerant;

an inner container of metal in which the heat exchanger and nozzle are permanently secured;

a Dewar flask in which the inner container is removably fitted, said flask having an inner wall for supporting a heat load on its outer surface;

a flowable heat-conducting heat transfer material disposed between the inner surface of said inner wall of the flask and the opposed outer surface of the inner container; and

a body of absorbent material disposed within the inner container for absorbing liquid refrigerant from the nozzle, said material being disposed for indirectly exchanging heat with a load on said outer surface of the inner wall of the flask. 

1. Cryogenic cooling apparatus comprising: a generally tubular heat exchanger providing two paths, in one of which refrigerant gas from a supply under pressure is supplied to a Joule Thomson expansion nozzle and in the other of which the low pressure gas is returned; a valve member cooperating with said nozzle to vary its effective area automatically for controlling the flow or refrigerant; an inner container of metal in which the heat exchanger and nozzle are permanently secured; a Dewar flask in which the inner container is removably fitted, said flask having an inner wall for supporting a heat load on its outer surface; a flowable heat-conducting heat transfer material disposed between the inner surface of said inner wall of the flask and the opposed outer surface of the inner container; and a body of absorbent material disposed within the inner container for absorbing liquid refrigerant from the nozzle, said material being diSposed for indirectly exchanging heat with a load on said outer surface of the inner wall of the flask. 